A287221 Number of twice-crossing partitions on n nodes.

0, 0, 0, 0, 8, 42, 168, 760, 2418, 10490, 30842, 131676

Offset

0,5

Comments

The meandric numbers A005316 are the numbers of paths which cross themselves 0 times.

This sequence is the number of paths that must cross themselves exactly twice.

Links

Table of n, a(n) for n=0..11.

B. W. J. Irwin, On the number of k-crossing partitions

Example

a(4) = 8; this is from the partitions
(2,1,4,3), (2,4,3,1),
(3,2,4,1), (3,4,2,1),
(4,1,3,2), (4,2,1,3),
(4,2,3,1), (4,3,1,2).

Crossrefs

Cf. A005316, A287220.

Sequence in context: A319235 A341764 A256861 * A119965 A229729 A055082

Adjacent sequences: A287218 A287219 A287220 * A287222 A287223 A287224

Keyword

nonn,more

Author

Benedict W. J. Irwin, May 22 2017

Status

approved